Telephone terminal equipment



Sept. 18, 1951 H. JFFERSON 2,568,397

TELEPHONE TERMINAL EQUIPMENT Filed' Feb. 19, 1948 2 Sheets-Sheet l i '0W/D07 l INVENTOR Harold Jef-z'zsozl BY ,ff-9' l D ATTORNEY Sept. 18, 1951 H. JEFFERSON 2,568,397

Y TELEPHONE TERMINAL EQUIPMENT Filed Feb. 19, 1948 2 Sheets-Sheet 2 lrog effersoll ATTO R N E r Patented Sept. 18, 1951 EfD' QTFEI'CEJ t TELEPHONE TERMIN ALAVEQUIPMENT Harold-f Jefferson,A Stockholm, Sweden, assignor to. Riadi'o Corporation of "America, `.aw-corpora"- tion of Delaware# Application,February- 19, 1948,1SjrialNo. 9541.-,

In'G'reatfBritain February 20,1947" Y This invention relates -to telephone terminar equipment and* moreparticularly 4to terminal equipment for the-"kind comprising a' transmitting channelfand- 'alreceiving channel associated with" asubscribersrline. Telephone terminal equip-- mentiof this kind commonly-comprises atrunk or other? subscribers telephone liney associated throughahybridfcoil-set Witha radio transmit-r ter-anda' radioreceiver so 'thata subscriber on-` minalequipment of :thefkind lreferred to'is pro'- vide'df'in the receiving channel with a 'network'l having three selectable conditions of attenuation cry-gain; in one'of which the receivingchannell is '-'off standard sensitivity, in: the second of *which the -receiving 'chan-nel is-'of 'reduced' sensitivityl and i-Y the-thirdof which the 'receiving-'channel is'sogreatly vreduced -or zeroI sensitivityand onef onftheptherfof th'ese three selectable conditionsr is automatically established "iii such mannerl that Whenvthetansmitter is operating the third conL ditionis established;l when the transmitter is not operating but no speech' is `being received, thel second'condition is'established, whilewhen the`- transmitter-'is'not operatingV and speechfis being received' the y first! condition is established;

'Il'ey invention' provides'l its -rnain advantagesl owing t the nowsdiscovered 'fact that Lfor physio= logicai'f'- orf psychological reasons` a considerable" practical fimprovement 'in the effectiveness 'of l'at telephonecommunication system is obtainedjiff during freception, noise is reduced Ain the'absence off'ispeechf. Tlieexplanations of'ithis improve-'-A men-t1 are; to some extent," 'conjectura1but itmay' well beftl'i'at theyy are-at least in part'to b'e found-5 inlltheffact that tif 'noise "is'reducedin thiel'intervals'i of vabsenceof speech, the earist restedfi-such' intervals4 and 'remains more sensi i tive t'speech occurring' between such' intervals; Wietherpr-notfth'is explanation is suiiiient;l eX; periment hasL shown'th'atlthe improvement does,` irl-'ffc-tfoccur andfirideed'experiment indicates' that'infoperatingia system iry accordance with`V this invention; it should-bek possible to--obtainA eiective operation With'a Vsignal-'noise-ratio substantially belowthatwhicl'i1 would otherwisebe required for the same resul-ts.

Preferably, in carrying out the invention; the netWork-lriaving-'three selectable conditionslof attenuationf or 1' gain: is constructed in any" convenient 1 manner known' per A se'v to'- establish af BOI 2.,., selected? one Yof those conditions in dependence upon ywhich of I'three predetermined control "voltages-is"appli'ed-thereto and these three voltages= are obtained-'by means =including^ a -scale'of-three; counter inturn controlledby-ian amplifier dei4 tector fedfr'om' the linput sidefof f the receiving@ charmels'4 The inventionisillstrated in the accompanying drawings, vin which:

Fig; l is '-ablocloY diagram of fa ftermirial 'station' accordig'to this invention;

Fig'. 2" is a-*detailed* circuit diagram of-*a por-- tion of Fig'fl and f F'igil Sissa-detailed circuit i diagram of Y anotherr portion of Fig. 1.

Referring-to Fig: lf'of the drawings, artrunki" orfoth'er vsubscribers/li'ne I is associated Withzafradiotransmitterf (not shown) 1 and with a radio receiver f (not shown) through ahybrid-'coil set 2? tl1e-vch`annel'-fto thi'etransmitter includingaf networkl lof 'controllable attenuation or'gair'rand the channelfirom the -receiver also including such? af-fn'etv'vorkA 4-. Themetwork'- IlY has three f conditions-0f attenu'ationor gain 'as already'described Wh'ilelthe network' 3 'has two conditions, namel34` one offstandardsensitivity and the other of" greatly freducedf or "zerosensitivityg A circuit'is branchedoiffrom `th'ev inputside -of the v receivM` ing channel toanamplieredetector fwhose ont-n put's fedi by means'of "the connection'- labeled' af b` asfonef-of 'thefcontrolling-'or input voltagesf tofa scale-nfitlfreey counter 'l-'which lcan provide,

by'j' means'f of-=f appropriatef output connections therein,` one of three-voltages'in dependenceupom the output` from fthe* ampliier detector; 5." The counter-6 controls,v via ananodefollowercoue. pli-ng'circuiti'L tlie-i network 4 in th'ereceiving channel lia-ving:4 th'ree-y conditions of "attenuation or# 'gain-; oneL-orftheother `of rthese conditions being established in dependence uponwhich of' thef-threevoltages'is 4provided by th'e counter'. Aisecondamplier -detector B-fisfe'd. from aV circuit branchedoil (from Y the= hybri'dcoilfsideof "the transmitting ch'anneiandth'e output of such"am-r vide; byfmeanssof appropriate outputconnec-A conditions being'established in idependence fupon" whichcoff the-two-vcltages`-is providedbyfth'e counter B* Y TheY` twor controllableegair networks 3 and-f 4 areg conveniently, push-pullamplifiers such-fas 3 these networks may be varied by variation of the bias applied at point g. Networks of this type are quite conventional and are well-known to those versed in the art. Although the tubes of the push-pull amplifier are shown as triodes in Fig. 2, variable-mu pentodes and tetrodes would in fact be used, the connections to the other electrodes being conventional,

Signals are tapped off the respective paths by the units 5 and 8 and rectified by the detectors therein. These detector circuits are quite conventional. Appropriate fractions of the rectified signal form the controlling input or bias voltages fed in at a, b and x, y to the scale-of-three circuit, shown in detail in Fig. 3.

The tubes Vr, VR and VT, herein termed the idle, receiving and transmitter tubes, respectively, are pentodes which have good suppressor grid control. The screen grids and screen grid connections of these tubes are not shown,

as they are straightforward D. C. connections. The tubes Vr, VR and VT are interconnected in such a way that any pair of them acts as a resistance-coupled multivibrator or trigger circuit and snaps from the on position to the off, or vice versa, only one tube of the three being on at any one time. The anode 9 of tube Vr is coupled through a resistor to the suppressor grid I0 of tube VR, and through another resistor to the control grid II of tube VT. The anode I2 of tube VR is coupled through a resistor to the control grid I3 of tube Vr and through another resistor to the suppressor grid I4 of tube VT. The anode I5 of tube VT is coupled through a resistor to the control grid I6 of tube VR and through another resistor to the suppressor grid I1 of tube Vr. The cathodes of al1 of the tubes are connected to ground and the negative side of a potential source, while the anodes of all the tubes are connected through individual resistors to the positive side of the potential source.

Points p and q (indicated as a bias connection in Fig. l) are connected through individual resistors to suppressor grid I1 and contro1 grid I3, respectively, such points being connected to a suitable bias potential source to bias the two grids of tube Vr. The bias potentials and resistors are so selected that in the absence of any signals the points p and q are sufficiently positive to maintain tube Vr in the conducting condition. The anode 9 is therefore at a low potential, and VT is cut off by means of the connection from such anode to control grid II, and VR is cut off by means of the connection from said anode to suppressor grid I0.

Points a and b are connected by individual resistors to suppressor grid I0 and control grid I6, respectively, and to the output'of amplier and detector 5, in order to supply signal from the receiving path of the system to the grids of tube VR. When a signal of greater than a predetermined amplitude appears in the receiving path, the output of detector 5 drives grids I l) and I6 positive until VR conducts. This causes a fall in the potential of anode I2, which is transferred through the resistance coupling to the control grid I3 of Vr to cut off V1. The potential of anode 9 rises as V1 is cut off, helping to bring up the voltage on suppressor grid I 0, thereby further aiding the turning-on of tube VR. Although at the same time the potential on control grid II of tube VT is brought up from its cut-off potential (due to the connection from anode 9 to such control grid) the fall or drop of potential at anode I2 (due to conduction in VR) holds down the potential on suppressor grid I 4 suciently to maintain VT non-conducting.

Thus, as so far described, the pair of tubes VR and Vr acts as a resistance-coupled multivibrator, snapping from the condition in which Vr is on and VR oi to the condition in which VR is on and Vr 01T; only one tube is on at any one time.

Points a; and y are connected by individual resistors to suppressor grid I 4 and control grid II, respectively, and to the output of amplifier and detector 8, in order to supply signal from the transmitting path of the system to the grids of tube VT. When a signal of predetermined strength appears in the transmitting path, the output of detector 8 drives grids I4 and II positive until VT conducts, provided that VR is not being held on by the output of detector 5. When VT conducts, the potential of its anode I5 drops, and this drop is transferred through the resistance coupling to the control grid I6 of VR to cut oil VR. The potential of anode I2 rises as Va is cut off, helping to bring up the voltage on suppressor grid I4, thereby further aiding the turning-on of tube VT. Although at the same time the potential on control grid I3 of tube Vr is brought up from its cut-off potential (due to the connection from anode I2 to such control grid) the fall or drop of potential at anode I5 (due to conduction in VT) holds down the potential on suppressor grid I1 suinciently to maintain Vr non-conducting. Thus, the pair of tubes VT and VR also acts as a resistance-coupled multivibrator, snapping from the condition in which VR is on and VT off to the condition in which VT is on and VR off; only one tube is on at any one time. In fact, any pair of the tubes Vr and VR and VT acts as a resistance-coupled multivibrator, snapping from the on condition to the off. When VR is conducting (signal in the receiving path and transmitter not operating) I need to apply a suitable voltage (say -2 volts) to the gain connection g (see Fig. 2) of the receiving network 4, to provide standard sensitivity for the receiving channel, and when Vr is conducting (transmitter not operating and no speech or signal being received) I need a smaller voltage (say -6 volts) which will cause an additional, loss in network 4 of about 12 db. To convert the anode potential drops to the required voltage rises, I use the anode follower circuit 1. One drive e1 for anode follower 'I is taken from a tapping point on the anode resistor of tube V1 and is applied through a resistor to the control grid of a pentode I 8. The second drive e2 for anode follower 'I is taken from a tapping pointV on the anode resistor of tube VR and is applied through a resistor to the control grid of tube I8. The drives e1 and e2 may be taken directly from the respective anodes 9 and I2 if desired.

With VT conducting (Vr and VR nonconducting, transmitter on) both e1 and e2 are large positive, and tube I8 draws large anode current to make its output point XR negative enough to cut off, or to produce about 50 db. loss in, receiver network 4, it beingv recalled that VT conducts when the transmitter is on or operating. Thus, when the transmitter is on the receiving channel is blocked or heavily attenuated. The drive es for anode follower I is taken from a tapping point on the anode resistor of tube VTV (it might be taken directly from anode I5) and is applied through a resistor to the control grid of a pentode I9. When VT is conducting, anode follower 'I' has a low input voltage es so that tube I9 is 01T and its output point XT is positive,

assess? bringing transmitter network 3 on or up to standard sensitivity.

With V1 conducting (noise reduction-idle condition) ei drops, decreasing the anode current in tube I8 and causing the potential at XR to rise; this partly opens receiver network 4, or reduces the loss lin such network to about 12 db. As VT is cut off (when V1 conducts), e3 rises, bringing on tube I9 and making its output point XT negative the transmitting anode follower l then cuts oi the transmitting network 3 or causes it to go to zero sensitivity.

If VR conducts (receiving condition), V'r is still non-conducting (network 3 therefore being of zero sensitivity), but e2 now drops, further decreasing the anode current in tube I8 and causing the potential at XR to rise further; this opens receiver network 4 completely, Yto provide standard sensitivity for the receiving channel. The drive e1 may be made smaller than e2 either by tapping down the anode load of tube V1 or by using a smaller load resistor than that used for tube VR.

To summarize the above operation, when the receiving channel is of standard sensitivity suitable for the reception of speech or of reduced sensitivity (the last condition occurs in the absence of received speech and in the simultaneous absence of transmission) the transmitting channel is blocked or heavily attenuated by network 3, While when the receiving channel is blocked or heavily attenuated by network 4 the transmitting channel is of standard sensitivity.

The hybrid coil 2 is of any suitable form well known per se and is consequently not shown in detail but represented merely by a block in the Fig. 1 diagram.

Although the invention is not limited to the use of any particular values oi attenuation, satisfactory results may be obtained by including in the receiving channel a three-condition network 4 which is arranged in one condition to introduce a very small standard loss or gain, in another condition to introduce an additional loss of about 12 bd. and in the last condition to introduce an additional loss of 50 db. or more.

Mutually interlocking control of the sensitivities of the transmitting and receiving channels as known per se in terminal equipment of the kind in question may be employed in conjunction with the present invention.

Having now particularly described and ascertained the nature of my said invention and in what manner the same is to be performed I declare that what I claim is:

1. In a communication terminal arrangement, a transmitting channel, a receiving channel, a voltage-responsive network of controllable gain in said receiving channel, means for producing a selected one of three predetermined control voltages in response to signal strengths in the transmitting and receiving channels, and means for applying the produced control voltage to said network to control the same to produce a corresponding predetermined gain in signals D355' ing therethrough.

2. An arrangement as dened in claim 1 wherein said rst named means produces one control voltage to give standard gain in said network in response to a predetermined signal strength in said receiving channel, a second cti-l trol voltage to give reduced gain in said network in response to a signal strength of less than a predetermined value in said receiving channel. and a third control voltage to give further reduced gain in said network in response to a predetermined signal strength in said transmitting channel.

3. An arrangement as defined in claim 2 wherein the standard gain is a condition of negligible attenuation, the reduced gain is a condition of approximately 12 db. attenuation, and the further reduced gain is a condition of at least 50 db. attenuation.

4. An arrangement as defined in claim 1 wherein the iirst-named means includes three electron discharge devices interconnected in such a way that any pair of them acts as a trigger circuit, the said discharge devices being controlled in part by an ampliiier detector fed from the input side of the receiving channel and in part by an ampliiier detector fed from the input side of the transmitting channel.

5. An arrangement as defined in claim 1 wherein said second-named means includes an anode follower circuit having its input coupled to said first-named means and its output coupled to said network'.

6. An arrangement as defined in claim 1 wherein a voltage-responsive network of controllable gain is inserted in the transmitting channel and wherein said rst-named means produces also a selected one of two predetermined control voltages in response to signal strengths in said channels, for controlling the gain of the network in the transmitting channel.

7. An arrangement as dened in claim 6 wherein said rst-named means produces a iirst set of control voltages to give standard gain in the receiving channel network and high attenuation in the transmitting channel network in 'response to a predetermined signal strength in said receiving channel, a second set of control voltages to give reduced gain in the receiving channel network and high attenuation in the transmitting channel network in response to a. signal strength of less than a predetermined value in said receiving channel, and a third set of control voltages to give further reduced gain in the receiving channel network and standard gain in the transmitting channel network in respense to a predetermined signal strength in said transmitting channel.

HAROLD JEFFERSON.

REFERENCES CITED The following references are of record in the le of this patent:

YUNITED STATES PATENTS Number Name Date 1,654,633 Bisbee Jan. 3, 1928 1,763,169 Mathes June 10, 1930 2.018.464 Nebel Oct. 22, 1935 2,205,142 Hoard June 18, 1940 2,278,828 Chardio Apr. 7, 1942 2,282,405 Herrick May 12, 1942 2,366,011 Donaldson Dec. 26, 1944 2,370,388 Baird Feb. 27, 1945 

